Sentries posted outside the mouth of an underground yellow jacket nest already were alert and agitated when Dan Leslie, clad in a white protective suit, bent to squeeze some powdered insecticide into their hole.

In moments, dozens more insects gathered above the entrance to the burrow, flying here and there as if confused and sensing cause for alarm.

Soon, their queen would be dead, the workers and larvae gone, too, as yellow jackets tracked the potent dust into their colony, spreading death from cell to cell in their underground stronghold.

“I usually tell the people about two days,” Leslie, a veteran technician with the Marin-Sonoma Mosquito and Vector Control District, said as he wrapped up his seventh yellow jacket call of the day.

Summer is high season in any year for these aggressive wasps, best known for disrupting picnics, parties and barbecues as they scavenge for food.

But there appears to be a bumper crop in the North Bay this year, likely a function of abundant winter rains and temperate spring conditions that allowed new colonies to proliferate around the region, creating hazards for humans and other species who may live nearby.

By Wednesday, the Marin-Sonoma Mosquito and Vector Control District had received 825 calls this year for help ridding yards of in-ground yellow jacket nests, compared with 576 at the same time last year, when a total of 1,219 calls were reported.

Peak month

August is typically the peak month for such calls, and already this month the tax-funded district has received 3½ times more yellow jacket calls than during the same eight-day period last year, Public Relations Director Nizza Sequeira said.

Though greater awareness of the century-old agency and the free services it offers may be another reason for the increase in yellow jacket calls, according to Sequeira.

But the weather this year likely had most to do with it. The mild, moist spring allowed a large crop of queens to establish nests that survived the first vulnerable weeks, said Eric Engh, an education program specialist.

Rain-fueled vegetation growth, in turn, spawned abundant wasp prey, allowing colonies to grow and continue to produce new larvae, churning out thousands of bugs per nest.

By late summer, as the region dries out, their natural food sources may diminish, prompting foraging that brings them increasingly in contact with humans, Engh said.

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If they sense a need to defend themselves or their nest, they can sting or bite, using mandibles made for chewing wood in nest building.

A person who is sensitive to their venom, or even allergic, could be at significant risk, Engh said.

Setting traps

Commercial lure traps may offer some relief by drawing yellow jackets away from an area where people are eating outside, for instance. If baited with a synthetic chemical called heptyl butyrate, which mimics the odor of fruit, it will not capture beneficial honeybees, Engh said.

While some people deploy traps in the spring to try to eliminate queens seeking rodent burrows and other cavities for nesting at that time of year, Engh said there are so many more queens than there are suitable sites that it makes no impact on the overall number of colonies.

But even a trap that accumulates hundreds of yellow jackets can have little impact on the health of a nest. That was the case on the Sebastopol property Leslie treated earlier this week.

Aaron Bates, 30, his dogs and his girlfriend, who is allergic, had all been stung by yellow jackets in their Elphick Lane yard over the past month by the time he called vector control a few days earlier.

He said the couple had hoped to resolve the problem on their own, short of killing the insects. So they tried a few different things, including a decoy wasps nest, without success.

Bates said they also used mud to try to seal the one nest they knew about, “hoping if we closed their door they would find a way out and go somewhere else.”

Bulb dusters

Finally, they called the vector control district, which sent Leslie two days later. That same morning, they found a second nest that was treated, as well.

The agency’s 16 technicians use bulb dusters to inject a tiny amount of powder containing a biodegradable insecticide called pyrethrin, which is derived from chrysanthemums flowers that stays in the nest, so it’s unlikely to pose a threat to other insects, like honeybees and other pollinators, Engh said.

“Yellow jackets are a nuisance when they forage on our food, and they become very defensive when we get near their nests. The safest way to eliminate yellow jackets is to call for professional help because one nest can contain several thousand wasps,” said Jamie Scott, president of the Mosquito and Vector Control Association of California. “You can avoid attracting yellow jackets to your yard by keeping food — including pet food — covered placing garbage in tightly sealed cans.”